Unfortunately a link to Rietveld is not available for review and inlined comments.

In sample() the selected set can be initialized to {n} to avoid additional checks in the loop for large population. In the branch for small population, non-empty pool can be extended ("pool.append(pool[-1])") to avoid additional check in the loop.

In choice() I would write the condition as "i == n > 0" to avoid indexing with negative index. Custom collection can has non-standard behavior with negative indices. This doesn't add additional cost in normal case.